A pig showing an elongate body with multiple surrounding external wiping ribs is set forth in U.S. Pat. No. 4,083,074. An alternative structure is shown in U.S. Pat. No. 4,069,525. These two patents set forth pipeline pigs having an elongate body with multiple pipe wiping ribs on the exterior. The term rib is used hereinafter to refer to a peripheral lip or fin extending radially outwardly, all for the purpose of wiping the interior of the pipe. Such pigs typically have elongate bodies and are shaped in the fashion of a bullet with a streamlined nose or point. Such pigs further use multiple ribs to assure that the pipe wall is adequately wiped. Pigs of this construction are typically hollow to reduce the cost of the pig. Moreover, a hollow pig of this type is generally lighter and less likely to wear flat on the bottom side. Pig weight is a determining factor in the formation of flat spots on the wiping ribs.
There is a risk of damage in the use of such a pig. Assume that a pig is travelling along a pipeline at a specified velocity urged along the pipe by a particular fluid flow rate and pressure behind the pig. Assume further that a shock wave overtakes the pig from the rear. The shock wave will typically be accompanied by a pressure rise of between 5% and 40%. In this representative situation, the propagation velocity of the shock wave in the pipeline is quite fast. This pressure wave front typically propagates very rapidly, and can travel the length of the pig in millisecond speed. In the typical case, the pressure pulse propagates through the cavity of the pig typified in the prior art to impinge on the nose of the pig very rapidly. The nose of the pig is then forced to yield or give. While the pig may well be moving, it does not move sufficiently fast in response to the rapidly propagated pressure wave travelling along the pipeline. This typically unduly loads the nose area of the internal cavity. Often, the rapid loading will blow the front nose out of the pig. That is, the pig will be ruptured by blowing out a portion of the nose area adjacent to the cavity.
The pig of the present disclosure overcomes this handicap. The pig of this disclosure is improved to avoid such a problem and difficulty. One approach to avoidance of this problem is to simply do away with the cavity. If the cavity is sealed and hence becomes a pressurized cavity, the pressure in the cavity must be periodically adjusted. While it might be initially adjusted to match the nominal pressure in the pipeline, this is usually inadequate to accommodate pressure surges. Rather, pressure surges will shrink the pig as the compressible fluid in the cavity shrinks. This is usually an unacceptable solution. A solid pig body is an alternate solution. However, this markedly increases the weight of the pig and cost also. As the pig weight increases, there is an increased tendency to wear a flat face on the bottom of the pig, thereby destroying its circular construction and permitting leakage past the pig in the pipeline.
This improved construction overcomes these severe limitations. It provides a pipeline pig with a hollow cavity and hence a lighter pig body. The pig body cavity is filled with lightweight foam. The foam is accessed through a constricted opening in the rear of the pig to fill the cavity. After the pig body has been filled with foam, the cavity defines a lightweight body. Moreover, the foam filling the cavity markedly retards pressure shock waves travelling along the pipe. With this in view, the present apparatus is described in general terms as comprising an elongate hollow bullet shaped pig body. It has a number of peripheral ribs on it for wiping the interior of the pipe. It has a transverse cup-like rear face which is urged by the pressure gradient in the pipe. The body is hollow, and is filled with foam. A small opening or constricted passage at the rear of the body admits fluid under pressure. The restriction of the opening has the preferred form comprising a threaded plug having a relatively small orifice in it.